There is known a surface acoustic wave device as an example of acoustic wave devices using acoustic waves. The surface acoustic wave device is equipped with a comb-like electrode formed on a surface of a piezoelectric substrate. The comb-like electrode is composed of a metal strip that excites, receives, resonates, or reflects the surface acoustic wave. The surface acoustic wave device has a small size and a light weight and is capable of greatly attenuating signals outside a given frequency passband. Thus, the device is used as a filter installed into a wireless apparatus such as a cellular phone terminal.
The advancements in high performance of cellular phones require an improvement in the temperature characteristic of the surface acoustic wave device and a reduction in the temperature dependence of the passband frequencies of the filter and/or a resonance frequency of a resonator. However, a piezoelectric single-crystal material having a large electromechanical coupling factor such as lithium tantalate (LiTaO3) lacks temperature stability.
Japanese Patent Application Publication No. 2004-343359 describes a technique for bonding a lithium tantalate substrate, as an element substrate on which an surface acoustic wave element is formed, on a supporting substrate composed of a sapphire substrate. Japanese Patent Application Publication No. 2002-9584 describes the following. The supporting substrate and the element substrate are formed by lithium tantalate substrates. The X-axis of the element substrate is se to the propagation direction of the acoustic wave of the surface acoustic wave element. The Z-axis of the element substrate is set to the axial direction of the supporting substrate parallel to the propagation direction of the acoustic wave.
It is to be noted that, however, in the technique described in Japanese Patent Application Publication No. 2004-343359, a bulk wave may be reflected on a boundary face between the sapphire substrate and the lithium tantalate substrate, leading to an occurrence of a spurious response. This results from a difference in acoustic impedance between the sapphire substrate and the lithium tantalate substrate. The acoustic impedance depends on the acoustic velocity in each substrate and the substrate density. The technique described in Japanese Patent Application Publication No. 2002-9584 has a problem due to a large deference in linear thermal expansion coefficient between the supporting substrate and the element substrate in the propagation direction of the acoustic wave. The large difference results in thermal stress, which may deform the substrates. Paragraph 0008 of the above Patent Document describes bonding between the supporting substrate and the element substrate by Van der Waals force. Thus, the influence of the supporting substrate makes it difficult to suppress the temperature dependence of the surface acoustic wave element formed on the element substrate. A heat treatment is performed at 250° C. as described in paragraph 0009 of the above Patent Document in order to further strengthen the bonding between the supporting substrate and the element substrate. This treatment may greatly deform the substrates depending on the difference between the linear thermal expansion coefficients in the X-axis and Z-axis directions. The setting of the Z-axis parallel to the propagation direction of the acoustic wave as described in Japanese Patent Application Publication No. 2002-9584 may greatly increase the difference between the linear thermal expansion coefficients. Thus, the heat treatment for forming the acoustic surface wave element on the element substrate may deform and warp the substrates. In a case where the acoustic surface wave element is formed with the substrate being deformed, the acoustic surface wave elements formed on the substrate may not have uniform characteristics.